A vertical cavity surface-emitting laser (VCSEL) element emits light in a direction perpendicular to a substrate of the element. VCSEL elements are gaining increasing attention because they are superior to edge-emitting semiconductor laser elements in terms of cost, power consumption, size, suitability for two-dimensional devices, and performance. The edge-emitting semiconductor laser elements emit light in a direction parallel to a substrate.
A surface-emitting laser element typically includes a confinement structure for improving current inflow efficiency. The confinement structure may be formed by selective oxidation of Al (aluminum) and As (arsenic) layers (which may be hereafter referred to as an “oxidized confinement structure”; see Patent Document 1, for example). Specifically, a mesa of a predetermined size having a to-be-selectively oxidized layer of p-AlAs exposed on the sides is formed. The mesa is then placed in a high-temperature water-vapor atmosphere so as to selectively oxidize Al from the sides, thereby leaving an area of the selectively oxidized layer at the center of the mesa that is not oxidized. The un-oxidized area forms a current passage area through which a drive current for the surface-emitting laser element passes. In this way, a current confinement structure can be readily obtained.
The refractive index of the Al oxidized (AlxOy) layer (which may be hereafter referred to as an “oxidized layer”) is about 1.6, which is lower than the refractive index of a semiconductor layer. As a result, a refractive index difference is caused in a lateral direction of the cavity structure, so that light can be confined in the central portion of the mesa. In this way, the light-emitting efficiency can be improved, and advantageous properties such as a low threshold current value and high efficiency can be realized.
A surface-emitting laser element may be utilized as a light source in an optical system of a printer (with an oscillating wavelength in a 780 nm band); a light source for writing in an optical disk unit (with an oscillating wavelength in a 780 nm or a 850 nm band); or a light source in an optical transmission system, such as a LAN (Local Area Network) using optical fibers (with an oscillating wavelength in a 1.3 μm or a 1.5 μm band). The surface-emitting laser element may also be utilized as a light source for optical transmission between or within boards, between LSI (Large Scale Integrated circuit) chips, or within an LSI chip.
In these fields of application, the light emitted by the surface-emitting laser element (which may be hereafter referred to as “emitted light”) is often required to have a circular cross-sectional shape. In order to obtain a circular cross-sectional shape of emitted light, high-order lateral mode oscillation needs to be prevented.
For example, Patent Document 2 discusses a technique for controlling lateral mode oscillation by forming an optically transparent film on an output surface and providing a reflectance difference between a central portion and a peripheral portion of a light-emitting area.
Single mode output is greatly influenced by the size of the current passage area. When surface-emitting laser elements are mass-produced by layering plural semiconductor layers on a substrate having a large area measuring 3 or 4 inches in diameter, for example, variations may be caused in the size of the current passage areas in the plane of a layered body of the semiconductor layers depending on a temperature distribution or water-vapor flow conditions.
Patent Document 3 discloses a surface-emitting laser array apparatus in which dummy elements that are not actually used as surface-emitting laser elements are disposed around a two-dimensional array of surface-emitting laser elements that are actually used. The dummy elements have the same post structure as the actually used surface-emitting laser elements. In this surface-emitting laser array apparatus, the distance between adjacent surface-emitting laser elements, the distance between adjacent dummy elements, and the distance between the surface-emitting laser elements and the adjacent dummy elements are made uniform.
However, the condition of layering of the semiconductor layers may differ among a plurality of layered bodies. Thus, variations may be caused in the size of the current passage area among the plural layered bodies even if their to-be-selectively oxidized layers are selectively oxidized under the same oxidizing conditions (including temperature and duration of the oxidizing process).
Patent Document 1: U.S. Pat. No. 5,493,577
Patent Document 2: Japanese Patent No. 3566902
Patent Document 3: Japanese Patent No. 3965801